Field of the Invention
The present invention relates to a marine 3D seismic survey apparatus. More particularly, it relates to a foldable-fixing type 3D seismic survey apparatus for a small ship, whereby the apparatus is able to fix relative positions of seismometers to locate each other with a small number of GPS devices, allows a survey ship to vertically and horizontally extend, and can be easily loaded and transported in a predetermined space on a vehicle by being folded, and a method of 3D seismic survey using the foldable-fixing type 3D seismic survey apparatus.
Description of the Related Art
In general, a marine seismic survey is performed while a survey ship travels with a source generating seismic waves and a streamer equipped with a seismometer sequentially towed behind the stern of the survey ship and periodically generates seismic waves from the source. Further, the marine seismic survey obtains marine information by analyzing the seismic waves reaching the seismometer after being reflected from the see floor.
For such a seismic survey, equipment such as an OBC (Ocean Bottom Cable Type) streamer disclosed in Korean Patent Application Publication No. 10-2012-0076952, a streamer having blades, a protective case, a retainer, and an undersea level maintainer etc. disclosed in Korean Patent Application Publication No. 10-2013-0134822, and a multi channel seismic survey apparatus disclosed in Korean Patent No. 10-1016014 are used, and a technology of converting data obtained from received seismic waves into an ODCE file disclosed in Korean Patent No. 10-1230040 is applied.
Seismic surveys are classified into a large-ship survey and a small-ship survey by the sizes of ships that are used. Further, when there is one streamer that is an array of seismometers, it is called a 2D survey, and when there are two or more streamers, it is called a 3D survey.
FIGS. 1A and 1B are respectively schematic views of 2D and 3D seismic surveys.
A 2D survey, as shown in FIG. 1A, obtains information about a 2D cross-section for a virtual plan including a source and a streamer, but a 3D survey, as shown in FIG. 1B, obtains 3D information from the point of time when acquiring data. Accordingly, the 3D survey can image complicated structures, as compared with the 2D survey, but requires a large ship to tow many pieces of equipment and obtain accurate location information of the equipment.
However, using a large ship may be impossible to perform a normal survey for areas with a small depth of the sea or for places where fishing nets, fishing implements, and ships frequently come and go. Therefore, it may be technically meaningful to obtain a 3D image through a small-ship survey.
Such an attempt has been made at abroad and FIGS. 2A and 2B show an example of a 3D seismic survey apparatus using a small ship in the related art, disclosed in VHR marine 3D seismics for shallow water investigations Some practical guidelines (Springer 2005 Tine Missiaen). A 3D seismic survey apparatus using a small ship, as shown in FIG. 2A, acquires 3D seismic survey data while towing several 8 m streamers having a 2 m gap therebetween.
FIG. 2B shows a survey process using a 3D seismic survey apparatus for a small ship that has this configuration. In FIG. 2B, the upper picture shows a survey on a river and the lower picture shows a survey at sea. As can be seen from the two pictures in FIG. 2B, the gaps between the streamers are not uniform as indicated by the arrows. This non-uniformity of streamers occurred around seacoast areas with currents, so the apparatus failed to obtain a precise 3D seismic image.
Misseian et al. who conducted the survey explained the reason, saying the apparatus failed to maintain the gaps between seismometers uniform due to currents around the seacoast. This may be considered reasonable because although a survey using a large ship can be performed quickly at a speed of about 5 knots as compared with using a small ship (2˜3 knots generally for a small ship), large ships can strongly tow streamers of several to hundreds of kilometers, and the relative positions between seismometers can be fixed by the tension in the streamers themselves. However, it is difficult to expect this effect from short streamers that small ships can tow.
Accordingly, this problem should be solved for a 3D seismic survey using a small ship.
The applicant(s) of the present invention has proposed ‘3 dimension seismic exploration apparatus and method in small ships’ as disclosed in Korean Patent Application No. 10-2014-0152208, in which the apparatus includes a seismic wave generator towed behind the stern of a small ship, a pair of support bars extending the small ship in the traveling direction at the rear portion of the seismic wave generator by being connected to the stern of the small ship, and a plurality of streamers coupled between the support bars and arranged in the traveling direction of the small ship, in order to perform a 3D seismic survey using a small ship by maintaining uniform gaps and undersea positions even for short streamers and to perform an accurate 3D seismic survey using a small ship by fixing short streamers at predetermined positions with respect to seismometers and the seismic wave generator even against currents.
This method has the advantage that it is possible to largely extend the streamers in the survey direction in a survey using a small ship, but four or more GPS devices are basically required to find out the relative positions of the seismometers. Thus, in terms of an electric/electronic aspect of system, a signal processing system having a large memory is required for such GPS devices. That is, a large processing system is required for many GPS devices for locating seismometers due to the flexibility of the streamers when a current is severe, and the system is not integrated with the seismometer devices, thus the system is likely to need to be disassembled for transporting and stowing and assembled again for surveying later.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.